CoAP Alive Message
draft-castellani-core-alive-00

Abstract

In the context of a Constrained RESTful Environment (CoRE), hosts could frequently be energy-constrained and be turned off the vast majority of time for energy-saving purposes.

In the case of a CoAP server, while it is offline, it is neither available to serve requests. Clients desiring to access its resources have no way to understand when they will find it up again.

This specification provides a simple new message that gives to a CoAP server the ability to signal its current availability in the network.

Status of this Memo

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This Internet-Draft will expire on September 28, 2012.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Alive (ALV) Message Type
• 3. Examples
• 4. Acknowledgements
• 5. References
• 5.1. Normative References
• 5.2. Informative References
• Authors' Addresses

1. Introduction

In the context of a Constrained RESTful Environment (CoRE), hosts could frequently be energy-constrained and be turned off the vast majority of time for energy-saving purposes.

In the case of a CoAP server, while it is offline, it is neither available to serve requests. Clients desiring to access its resources have no way to understand when they will find it up again.

This specification provides a simple new message that gives to a CoAP server the ability to signal its current availability in the network.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. Alive (ALV) Message Type

This specification defines a new message type for the CoAP message-layer (see Section 4.4 of [I-D.ietf-core-coap]). The type of a message is specified by the T field of the CoAP header.

An "Alive" message (ALV) indicates that a CoAP server is up and ready to serve requests. Alive shares T value with the Non-Confirmable (NON) message type, but is univocally distinguishable by the fact that it MUST be empty.

When a client receives an ALV message from a server, if it is interested in any resource served by it, the client SHOULD try sending a request to it since the Alive message provides an indication of its current availability.

3. Examples

Figure 1 shows a sample use case where a server notifies its wake up to clients using multicast.

    
C1  C2  C3    S
|   |   |     . server is sleeping
|   |   |     .
|   |   |     .
|   |   |     .
|   |   |     . server wakes up
|<--|<--|<----| ALV MID=0xfefe
|   |   |     |
|   |   |     | CON MID=0x1234
|------------>| GET /a
|   |   |     |
|   |   |     | ACK MID=0x1234
|<------------| 2.05 "A"
|   |   |     .
|   |   |     . server goes sleeping again
|   |   |     .
|   |   |     .
|   |   |     .
    
        

Figure 2 shows a sample application on how the Alive message can fix the "Observer Model" problem discussed in Section 4.2.3 of [I-D.arkko-core-sleepy-sensors].

    
C1  C2  C3    P    S
|   |   |     |    . server is sleeping
|   |   |     |    .
|   |   |     |    . CON MID=0x1234
|   |   |     |    . GET
|   |   |     |    . Proxy-URI: coap://S/temp
|------------>|    . Observe: 0
|   |   |     |    .
|<------------|    . ACK MID=0x1234
|   |   |     |    .
|   |   |     |    . (time passes..)
|   |   |     |    .
|   |   |     |    . server wakes up
|   |   |     |<---| ALV MID=0xfefe
|   |   |     |    |
|   |   |     |    | CON MID=0xabcd
|   |   |     |    | GET /temp
|   |   |     |--->| Observe:0
|   |   |     |    |
|   |   |     |    | ACK MID=0xabcd
|   |   |     |    | 2.05 "22.5 C"
|   |   |     |<---| Observe: 789
|   |   |     |    .
|   |   |     |    . CON MID=0xabce 
|   |   |     |    . 2.05 "22.5 C"
|<------------|    . Observe: 3013
|   |   |     |    .
|------------>|    . ACK MID=0xabce
|   |   |     |    . 
|   |   |     |    . (time passess..)
|   |   |     |    . 
|   |   |     |    . resource state changes
|   |   |     |    . NON MID=0xfeff
|   |   |     |    . 2.05 "21.5 C"
|   |   |     |<---| Observe: 1384
|   |   |     |    .
|   |   |     |    .
|   |   |     |    . NON MID=0xabcf 
|   |   |     |    . 2.05 "22.5 C"
|<------------|    . Observe: 3013
...
    
        

4. Acknowledgements

TBD

5. References

5.1. Normative References

5.2. Informative References

Authors' Addresses